In the nature, boron isotope (δ11B) varies in a large range and the isotopic composition of boron differs significantly in different environmental and geological process. For this reason, boron isotopic composition is widely applied in the fields of crust-mantle evolution, mineral deposits, hydrochemistry, environmental geochemistry, marine environment and paleoenvironment. With the improvement in the determination methodology and analyzing accuracy, boron isotope as a sensitive and reliable indicator has been employed in various scientific fields, paleooceanography, paleoenvironment, environmental monitoring, pollution sources identification etc. Boron isotope is the most promising tools in the research field of geochemistry in the recent two decades.
Due to the remarkable indicative significance of boron isotopic composition for the changes of environmental and geological processes, the purification-separation procedure and the analytical methods for boron isotopes have been significantly developed and improved to deal with natural samples with rich organic matter, complex matrix and low boron content.
At present, the mass-spectrometric (MS) techniques for determining boron isotopic composition mainly include Positive Thermal Ionization Mass Spectrometry (Cs2BO2+-PTIMS), Negative Thermal Ionization Mass Spectrometry (BO2−-NTIMS), Inductively-Coupled-Plasma Mass Spectrometry (ICP-MS), Multi-Collector Inductively-Coupled-Plasma Mass Spectrometry (MC-ICP-MS) and Secondary-Ionization-Mass-Spectrometry (SIMS). The main features and research progress of these determination methods has been compared as shown in Table 1 (Aggarwal J. K. et al., Precise and accurate determination of boron isotope ratios by multiple collector ICP-MS: origin of boron in the Ngawha geothermal system, New Zealand, Chemical Geology, 2003, 199, 331-342).
Because of some inherent disadvantages related to different measurement techniques, such as, relatively large quantity of boron required for PTIMS, larger measurement uncertainty for NTIMS, higher random errors for ICP-MS, poor internal precision and crucial dependence on sample matrix for SIMS (Jugdeep K. et al. Boron Isotope Analysis A Review, Analyst, 1995, 120, 1301-1307, Hemming N. G., Hanson G N., Boron isotopic composition and concentration in modern marine carbonates. Geochimica et Cosmochimica Acta, 1992, 56, 537-543). there is no any single instrument could satisfy the determination of boron isotopic composition for all kinds of sample. The accurate determination of boron isotopic composition in the natural samples with low boron content, complex composition and rich organic and biological matters is still a big challenge. In mineral resource and eco-environmental chemistry fields, boron isotopic composition is used to trace the origin of ore formation, pH change of seawater, CO2 concentration in atmosphere, climatic evolution, changes of sea level, and origin and evolution of salt lakes, but ICP-MS method can not be used in these studies as an accurate method because the obtained 11B/10B ratio has a low accuracy. Therefore, the determination of boron isotopic composition in natural samples mainly adopts PTIMS & NTIMS and MC-ICP-MS methods.
TABLE 1Comparison of various MS techniques for determination of boron isotopic compositionMS Pre-SampleAccuracyDis-techniquetreatmentsize(%o)AdvantageadvantageMC-Complete250 ng±0.2Small sample Expensive ICP-separationsize, high instru-MSaccuracy and mentshigh analysis speedHR-No250 ng±2High Low ICP-analysis accuracy, MSspeed and no and serious need of pre-memory treatmenteffectQuad-No100 ng±14High Extremely ICP-MSanalysis low deter- speedminationaccuracyLA-MC-NoNano-<1In-situ Instrument ICP-MSgram levelanalysis of frac-solid sampletionationand driftPTIMSComplete1 μg±0.4High High separationaccuracysamplepurity andlong timeNTIMSNeed pre-10 ng±0.8Small Low treatmentsample sizeaccuracySIMSNo±4In-situ Inability to analysis of analyze solid sampleliquid and vapor phase
At present, the method of Cs2BO2+-graphite-PTIMS is well employed by many laboratories in the world, which first was introduced by Y K. Xiao et al (Y. K. Xiao, Beary E S, Fassett J D. Int. J. Mass Spectrom. Ion. Proc. 85 (1988)203) who found the intensity of Cs2BO2+ emitted from Cs2B4O7 can be increased to 2-orders of magnitude and when loading graphite on the filament in TIMS. During instrumental determination, a single central Faraday cup is used to collect m/e309 and m/e308 ions in the mode of peak jumping (i.e. dynamic single-collection method). According to Equation (Eq 1), the boron isotopic ratio 11B/10B is obtained based on 309/308 ratio. It is estimated as one of the best methods for the determination of boron isotopic composition with the highest precision of 0.1‰ (1σ) at the optimal condition (K. Jugdeep et al.).11B/10B=R309/308−0.00079  (Eq 1)
However, this method has considerable limitations in the determination of natural samples with low boron content. Its remarkable defects include: (1) Under the condition of low boron content (<1 μg), Cs2BO2+ ions can hardly maintain steady emission and are highly prone to decay in a short time; (2) The data acquisition in the mode of dynamic peak jumping is slow, and the ion signal has attenuated completely before completing 10 Cycles/10 Block 100 data acquisition for a single sample. Moreover, during dynamic data acquisition, when the magnetic field of mass spectrometer jumps to peak 308 (referring to the peak of Cs2BO2+ ion with m/e 308 in this Description) after data acquisition of peak 309 (referring to the peak of Cs2BO2+ ion with m/e 309 in this Description), the ion intensity has been changed and the provided 309/308 ratio is not true. As a result, the determined 11B/10B ratios deviate from the true value.
Many researchers have tried for long time to use simultaneous static collection of peak 309 and peak 308 to improve the precision for determining boron isotope ratio by TIMS-dynamic jumping of peak 309 and peak 308. They face the following major technical difficulties: (1) As the mass to charge ratio (i.e. m/e) of Cs2BO2+ ions is large (m/e=308 and 309), the separation of the two ions needs a larger radius of sector magnetic field in the mass spectrometer according to the equation for deflection of charged ions by magnetic field in the mass spectrometer (Eq 2); (2) When the ratio of peak 309 and peak 308 of Cs2BO2+ ions is collected to determine 11B/10B, the gap between the two parallel Faraday cups in order to full collection of m/e 309 ion and m/e 308 ion must be very small as the relative mass difference of the two detected ions is very small, only 0.0032 as obtained from Equation (Eq 3). In the recent years, the newly developed TIMS instruments have greatly improved the ionization efficiency of ion sources, the determination accuracy and sensitivity of isotopic ratio and the update of instrument control hardware and data analysis software, but it does not have much improvement in mass dispersion and is still unable to use the Faraday cups provided for commercial TIMS instruments to conduct simultaneous full double-collection determination of m/e 309 (133Cs211B16O2+) m/e 308 (133Cs210B16O2+) ions under normal condition.
                    R        =                                                            2                ⁢                U                                            H                2                                      ×                          m              e                                                          (                  Eq          ⁢                                          ⁢          2                )            
Where: R is ion deflection radius; U is electric field voltage; H is magnetic field strength.
                              Δ          ⁢                                          ⁢          m                =                                                            m                2                            -                              m                1                                      m                    =                                    1              309                        =            0.0032                                              (                  Eq          ⁢                                          ⁢          3                )            
To solve these technical problems, the general method for the TIMS instruments which may adjust high voltage is to reduce the deflection radius of Cs2BO2+ ions with a very large mass charge ratio in the sector magnetic field through reducing the high voltage of the ion source accelerator (for example, reducing from the set of 10.0 kV to 8.0 kV, or from the set of 8.0 kV to 5.5 kV), and increase the flight dispersion angle of m/e 309 and m/e 308 ions, and apply simultaneous collection of m/e 309 and m/e 308 ions through adjusting the two parallel Faraday cups (A. Deyhle, Improvements of boron isotope analysis by positive thermal ionization mass spectrometry using static multicollection of Cs2BO2+ ions. International Journal of Mass Spectrometry, 2001, 206, 79-89). For the newly developed TIMS instrument, the manufacturer might fix two parallel cups seamlessly during assembly of Faraday cup collector hardware with special requirements of scientific research to achieve simultaneous collection of the two m/e 309 and m/e 308 ions.
However, the current two methods that might perform the simultaneous collection stressed above are limited to specific models or special TIMS instrument and are not universally applicable. When the above techniques are applied on other TIMS instruments, they appear the following limitations: (1) Some models of TIMS can not change the high voltage of the ion source accelerator through instrument control software and operation panel and can not apply static multi-collection determination through reduction of high voltage; (2) As for the instrument with two Faraday cups fixed together by the manufacturer, the distance between the fixed Faraday cups is unadjustable. This also limits the application of the fixed group of Faraday cups group when it collects the detected ions during determination of the isotopes of other elements.
To solve this problem, The present invention increases the flight deflection angle of m/e 309 and m/e 308 ions in the ion flight channel through adjusting and changing the parameters of Zoom Optics in TIMS according to the focusing principle of the ion source of the mass spectrometer, and meanwhile select the two cups with the largest deflection angle, set their distance and simultaneously collect the two ions. After optimizing the two parameters of Focus Quad and Dispersion Quad in Zoom Optics, the perfect shape and full superposition of peak 309 and peak 308 are achieved after setting a mass number for the center cup of the Faraday collector. The method of the present invention successfully establishes accurately determination of boron isotopic composition by PTIMS-double Faraday cup static collection without changing high voltage parameters and Faraday cup hardware setting conditions.